Cellular telephone

ABSTRACT

A cellular telephone ( 100 ) that searches for a destination base station in accordance with a handoff instruction receives a soft handoff instruction or a hard handoff that is able to specify a first search window size. When the hard handoff instruction is received, a search block ( 70 ) is controlled so that a window control block ( 83 ) searches for pilot signals from neighboring base stations using a search window having a size larger than one of the first search window size and a third search window size that is used when the soft handoff instruction is received.

TECHNICAL FIELD

The present invention relates to a handoff in a CDMA cellular telephonesystem.

BACKGROUND ART

A CDMA cellular telephone performs a handoff as the cellular telephonemoves from one cell to another, in order to switch the communicationchannel from a currently communicating base station (also referred to as“previous base station”) to a destination base station. There are twotypes of handoffs: a soft handoff and a hard handoff. When performingthe soft handoff, the cellular telephone starts communicating with thedestination base station using the same frequency as the frequencycurrently used, before closing the communication with the communicatingbase station. When performing the hard handoff, the cellular telephonestarts a communication, after closing the communication with thecommunicating base station, with the destination base station in adifferent base station group, having a different frequency allocation,or having a different frame offset.

The soft handoff is performed based on pilot intensities of neighboringbase stations. A pilot intensity is an intensity of a pilot channelcommon to the neighboring base stations, but outputted at a differentspread timing from each base station, and also indicates a ratio ofreception pilot energy in an entire reception energy. The cellulartelephone measures the pilot intensities of the neighboring basestations on a regular basis, and notifies the communicating base stationof the measured intensities. If any of the neighboring base stations isnoted to have a sufficient pilot intensity that is stronger than thecommunicating base station, the communicating base station instructs thecellular telephone to perform a soft handoff to switch to the notedneighboring base station. Because the cellular telephone has alreadyspecified the spread timing (a synchronization timing) of all of theneighboring base stations by measuring the pilot intensities, thecellular telephone can synchronize with the noted neighboring basestation as soon as the soft handoff instruction is received.

On the other hand, the hard handoff is for switching base stations thatare in different base station groups or have different frequencyallocations or different frame offsets, and a hardware structure doesnot allow the cellular telephone to measure a pilot intensity of thedestination base station before actually performing the handoff.Therefore, the cellular telephone measures the pilot intensity of thedestination base station only after the hard handoff instruction isreceived from the communicating base station, and specifies thesynchronization timing of the destination base station based on themeasurement. When measuring the pilot intensity of the destination basestation, the cellular telephone estimates the synchronization timing ofthe destination base station based on the synchronization timing of theprevious base station, and then sets a search window in accordance withthe estimated synchronization timing and measures the pilot intensityduring a period that the search window indicates.

As has been described, the search window in the hard handoff is setbased on the synchronization timing of the previous base station.Accordingly, when radiowaves from the destination base station delayconsiderably due to reflections and such, a pilot signal peak of thedestination base station falls outside the search window and thecellular telephone fails to acquire the synchronization timing of thedestination base station. Such a failure of the handoff would cease atelephone call.

DISCLOSURE OF THE INVENTION

In view of the above noted problem, the present invention aims toprovide a cellular telephone having an increased rate of successfulhandoffs in a CDMA cellular telephone system.

In order to achieve the above object, a cellular telephone according tothe present invention is such that the cellular telephone that searchesfor a destination base station in response to a handoff instruction,comprising: a receiving unit operable to receive the handoff instructionfrom a current base station, the handoff instruction being one of a softhandoff instruction and a hard handoff instruction that is able toinclude information indicating a first search window size; and asearching unit operable, when the received handoff instruction is thehard handoff instruction, to search for a pilot signal from aneighboring base station using a search window that is larger in sizethan at least one of the first search window size and a second searchwindow size that is predetermined for the soft handoff instruction.

The above problem does not occur in a case of the soft handoff because apilot search of the neighboring base stations is performed prior to anactual soft handoff and the actual handoff is performed only after thetiming of the pilot channel is acquired. On the other hand, in a case ofthe hard handoff, a pilot search of the destination base station is notperformed prior to an actual handoff, and accordingly the above problemcould easily happen. The present invention makes it possible to suppressfailures in hard handoffs even when a time delay between radiowaves fromthe communicating base station and the destination base station islarge, by absorbing the time delay using a large search window.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a structure of a cellulartelephone of an embodiment of the present invention.

FIG. 2 is a diagram illustrating a search window table.

FIG. 3A illustrates a usual hard handoff point.

FIG. 3B illustrates an unusual hard handoff point.

FIG. 4 illustrates a relation between radiowave intensities of the basestations A and B, and the search window.

FIG. 5 is a flow chart illustrating an operational procedure mainly of acontrolling unit 80.

BEST MODE FOR CARRYING OUT THE INVENTION

The following describes a preferred embodiment of the present inventionwith reference to drawings.

FIG. 1 is a block diagram illustrating a structure of a cellulartelephone of an embodiment of the present invention.

In the drawing, a cellular telephone 100 includes an antenna 10, a radiounit 20, a modulation/demodulation unit 30, a voice processing unit 40,a microphone 50, a speaker 60, a searching unit 70, and a controllingunit 80.

The radio unit 20 is for converting frequencies between a wireless part(between a base station and the antenna 10) and themodulation/demodulation unit 30. Based on an instruction from thecontrolling unit 80, the radio unit 20 is able to convert thefrequencies according to a frequency assignment to a traffic channel.

The modulation/demodulation unit 30 is made mainly of a modulationsubunit that performs a narrowband modulation and a spread modulation,and a demodulation subunit that performs a reverse-spread demodulation.

The voice processing unit 40 performs a CODEC processing of voices thatare inputted via the microphone 50 and outputted via the speaker 60.

The searching unit 70 includes such as a matched filter 71 and anintensity measuring unit 72, and performs a pilot search of neighboringbase stations while in a communication with a currently communicatingbase station, and a pilot search for a destination base station whenperforming a hard handoff.

The pilot search is an operation in which a search window is set inorder to search available multipath components of a pilot channel, i.e.multipath components that the cellular telephone 100 can use in order todemodulate a corresponding receiving traffic channel. A size of thesearch window is determined based on an instruction from thecommunicating base station. The pilot channel is a series of signalstransmitted from each base station. Using the pilot channel, thecellular telephone can acquire the timing of reception CDMA channel.Base stations are identified by different pilot PN series offsets.Specifically, the pilot channel is outputted from each of the basestations at 512 different timings, because a cycle of the PN series istwo to the fifteenth power. By identifying a particular timing of thepilot channel, the cellular telephone can identify a particular basestation that corresponds to the pilot channel.

The cellular telephone 100 searches pilot channels on the current CDMAfrequency allocation when performing the pilot search of the neighboringbase stations, detects the pilot channels of the neighboring basestations, and measures intensities of the detected pilot channels. Whena detected pilot channel from any of the neighboring base stations has asufficient intensity, the cellular telephone 100 notifies thecommunicating base station of a pilot intensity measurement result, andthe communicating base station instructs the cellular telephone toperform a handoff in response.

The cellular telephone 100 performs a synchronization acquisition of thedestination base station by the pilot search of the destination basestation in the hard handoff. The synchronization acquisition is anoperation so as to synchronize a timing of a spread code of the cellulartelephone with a spread code for the received signals.

The matched filter 71 performs a correlation detection based on aconvolution integral of the reception signals and PN series, while asignal that indicates a period in which the search window is open andthat is transmitted from a window controlling unit 83 is inputted.

The intensity measuring unit 72 measures the pilot intensity of thepilot channel by adding, to a result of the correlation detection by thematched filter 71, a proportion between a received pilot energy per chipEc and a total amount of spectral densities received via the multipathcomponents (noise and signals) Io. Further, the intensity measuring unit72 specifies the peak position by measuring the intensity, and detectsthe pilot channel timing.

The controlling unit 80 is made of such as a CPU, RAM, and ROM (notshown in the drawings). Various controls are performed by the CPUexecuting different programs stored in the ROM.

Functionally, the controlling unit 80 includes a handoff processing unit81, a frequency setting unit 82, the window controlling unit 83, asearch window memory 84, and a search window table 85.

The handoff processing unit 81 perform a handoff by controlling eachunit, upon reception of a handoff instruction from the communicatingbase station. More specifically, the handoff processing unit 81 firstspecifies a handoff type, a frequency assignment, a pilot PN seriesoffset index, search window size information, and such, by the handoffinstruction. The handoff type indicates either the soft handoff or thehard handoff. The frequency assignment indicates a frequency assigned tothe destination base station, and is not included in the instructionwhen the handoff type indicates the soft handoff. The pilot PN seriesoffset index indicates a number of the PN offsets (each contain 64 PNchips) per pilot of the destination base station. The search window sizeinformation indicates a size of the search window used for the pilotsearch, and is not always contained in the instruction.

Next, the handoff processing unit 81 controls the radio unit 20 so as toperform a frequency conversion by synchronizing the frequency of thespecified frequency assignment.

Further, the handoff processing unit 81 reads, from the search windowtable 85, a search window size corresponding to the received searchwindow size information, and stores the received search windowinformation and the read window size parameter in the search windowmemory 84.

Further, the handoff processing unit 81 notifies the window controllingunit 83 of the pilot PN series offset index and the handoff type.

The frequency setting unit 82 controls switching frequencies at theradio unit 20, according to the frequency assignment of the destinationbase station indicated in the handoff instruction analyzed by thehandoff processing unit 81.

The search window table 85 is a search window table as illustrated inFIG. 2. As shown in the drawing, the search window table includes thesearch window information on the left column and window size parameterseach corresponding to the search window information on the right column.Each of the window size parameters correspond to a number of PN chips ofa search window that is used when searching the pilot channel.

The received search window information and the corresponding window sizeparameter that is read from the search window table 85 by the handoffprocessing unit 81 are stored in the search window memory 84.

The window controlling unit 83 notifies the matched filter 71 of thetiming of the search window in the pilot search based on the searchwindow information and the corresponding window size parameter stored inthe search window memory 84. During communication, the windowcontrolling unit 83 performs the pilot search with a center of thesearch window size indicated by the window size parameter being set atthe pilot PN series offset of each of the neighboring base stations.

When the handoff type indicates the hard handoff, the window controllingunit 83 reads the search window information stored in the search windowmemory 84, increments a number in the search window information by 1,and read a window size parameter corresponding to the incremented numberof the search window information from the search window table 84. Then,the window controlling unit 83 performs the pilot search of thedestination base station with setting the pilot PN series offset of thedestination base station as a center of the search window with a sizeindicated by the window size parameter that is read from the searchwindow table 84.

By the above structure, in a case of the hard handoff, the cellulartelephone 100 performs the pilot search of a destination base stationusing a search window having a size larger than a size indicated by thewindow size parameter stored in the search window memory. Accordingly,the pilot search in the hard handoff is performed using a search windowwith a larger size than the window size for the soft handoff or thewindow size instructed by the communicating base station.

FIG. 3A illustrates a usual hard handoff point, and FIG. 3B illustratesan unusual hard handoff point.

In FIGS. 3A and 3B, when the cellular telephone performs a hard handoffand switches a communication from a base station A to a base station B,the cellular telephone sets a search window of a pilot search for thebase station B using radiowaves from the base station A as a base.Specifically, a search window timing is determined based on a time delayof the radiowaves from the base station A.

In FIG. 3A, the cellular telephone (a mobile device in the drawings) ispositioned about halfway between the base stations A and B, and a timedifference between a time delay α1 (a time delay of the radiowave fromthe base station A) and a time delay α2 (a time delay of the radiowavefrom the base station B) is not large. Therefore, the time difference isabsorbed in the search window size that has been instructed in the hardhandoff instruction by the base station A, or the search window sizeused in a previous pilot search.

On the other hand, in FIG. 3B, the cellular telephone (a mobile devicein the drawings) is positioned close to the base station A and remote tothe base station B, and a time delay of radiowaves from the base stationB becomes larger because there are more reflections against buildingsand such. Accordingly, a time difference between a time delay β1 (a timedelay of the radiowave from the base station A) and a time delay β2 (atime delay of the radiowave from the base station B) becomes larger thanthe case in FIG. 3A, and in some cases, the time difference cannot beabsorbed in the search window size that has been instructed in the hardhandoff instruction by the base station A, or the search window sizeused in a previous pilot search. When the time difference cannot beabsorbed, the cellular telephone fails to acquire pilot signals from thebase station B, and as a result, the telephone call stops because of afailure in the hard handoff.

In a case of a soft handoff, such a problem does not occur because thesoft handoff is performed after acquiring the timing of the pilotchannel by performing the pilot search of the neighboring base stationsin advance. On the other hand, hard handoffs are susceptible to such aproblem, because a pilot search prior to an actual hard handoff is notperformed. The present invention aims to prevent handoff failures evenin the case of hard handoffs, by making the search window larger so asto absorb the time difference even when the time difference between thecommunicating base station and the destination base station is large.

FIG. 4 illustrates a relation between radiowave intensities of the basestations A and B, and the search window.

In the drawing, a left-to-right direction indicates a time flow. A PN100indicates a pilot PN offset index of the base station A, and a PN 200indicates a pilot PN offset index of the base station B. During thecommunication with the base station A (whose frequency assignment is 25ch), the cellular telephone 100 detects a radiowave peak from theradiowave intensity of signals from the base station A, and obtains andstores the time delay β1 by calculating the detected peak and a basetime of the PN100. Upon reception of a hard handoff instruction to thebase station B (whose frequency assignment is 50 ch), the windowcontrolling unit 83 estimates a timing of the PN200 and a timing that isβ1 minutes behind the timing of the PN200, based on the base time of thePN100 and the time delay β1. Then based on the timing that is β1 minutesbehind the timing of the PN200, the window controlling unit 83 opens asearch window having a size larger than a size instructed by the basestation A or a size stored in the cellular telephone 100 in advance.Although the radiowave peak of the signals from the base station B isbehind by the time delay β1, the peak can be detected within the searchwindow because the search window is large.

FIG. 5 is a flow chart illustrating an operational procedure mainly of acontrolling unit 80.

When the handoff processing unit 81 receives a handoff instruction fromthe communicating base station, the handoff processing unit 81 specifiesa handoff type, a frequency assignment, a pilot PN series offset index,search window size information, and such, by analyzing the receivedhandoff instruction, and then transmits necessary information to thefrequency setting unit 82, the window controlling unit 83, and such(Steps S401 and S402).

The window controlling unit 83 identifies whether or not the handofftype is the hard handoff. If the handoff type is the hard handoff, theoperation proceeds to Step S404, and if not, proceeds to Step S405 (StepS403).

The window controlling unit 83 obtains, from the search window table 85,a search window size that is larger by 1 size than the search windowsize that is stored in the search window memory 84, or contained in thehandoff instruction (Step S404). Next, the window controlling unit 83controls the matched filter 71 so as to open a search window with thesize obtained in Step S404 at the timing estimated in a manner explainedusing FIG. 4, and have the matched filter 71 and the intensity measuringunit 72 perform the pilot search (Step S406).

On the other hand, if the handoff type is not the hard handoff, thewindow controlling unit 83 uses the search window size either stored inthe search window memory 84 or contained in the handoff instruction, andhave the matched filter 71 and the intensity measuring unit 72 performthe pilot search.

INDUSTRIAL APPLICABILITY

A cellular telephone of the present invention has a high success rate inhandoffs, and is useful for a CDMA cellular telephone that is requiredto have a high communication quality.

1. A cellular telephone that searches for a destination base station inresponse to a handoff instruction, comprising: a receiving unit operableto receive the handoff instruction from a current base station, thehandoff instruction being one of a soft handoff instruction and a hardhandoff instruction that is able to include information indicating afirst search window size; and a searching unit operable, when thereceived handoff instruction is the soft handoff instruction, to searchfor a pilot signal from a neighboring base station using a search windowof a second search window size, and operable, when the received handoffinstruction is the hard handoff instruction, to search for a pilotsignal from a neighboring base station using a search window that islarger in size than at least one of the first search window size and thesecond search window size that is predetermined for the soft handoffinstruction.
 2. A cellular telephone according to claim 1, wherein thesearching unit comprises: a storage subunit that stores informationindicating the second search window size; a judging subunit operable tojudge whether the received handoff instruction is the hard handoffinstruction; and a searching subunit operable to search for the pilotsignal from the neighboring base station, the search being performed (i)using the search window that is larger in size than at least one of thefirst and second search window sizes when the judgment is in theaffirmative, and (ii) using a search window having the same size as oneof the second search window size and a third search window size that isspecified by the received handoff instruction, when the judgment is inthe negative.
 3. The cellular telephone of claim 1 further including inthe searching unit a search window table setting a priority of searchwindow sizes from the smallest to the largest search window size and asearch window memory storage unit for storing search window sizeinformation corresponding to the search window table wherein thesearching unit automatically sets the search window size one size largerthan the first search window size when a hard handoff instruction isreceived.
 4. A method used for a cellular telephone that searches for adestination base station in response to a handoff instruction, themethod comprising: a receiving step of receiving the handoff instructionfrom a current base station, the handoff instruction being one of a softhandoff instruction and a hard handoff instruction that is able toinclude information indicating a first search window size; and asearching step of, when the received handoff instruction is the softhandoff instruction, searching for a pilot signal from a neighboringbase station using a search window of a second search window size, andwhen the received handoff instruction is the hard handoff instruction,searching for a pilot signal from a neighboring base station using asearch window that is larger in size than at least one of the firstsearch window size and the second search window size that ispredetermined for the soft handoff instruction.
 5. A program that has acellular telephone execute a search for a destination base station inresponse to a handoff instruction, the program comprising: a receivingstep of receiving the handoff instruction from a current base station,the handoff instruction being one of a soft handoff instruction and ahard handoff instruction that is able to include information indicatinga first search window size; and a searching step of, when the receivedhandoff instruction is the soft handoff instruction, searching for apilot signal from a neighboring base station using a search window of asecond search window size, and when the received handoff instruction isthe hard handoff instruction, searching for a pilot signal from aneighboring base station using a search window that is larger in sizethan at least one of the first search window size and the second searchwindow size that is predetermined for the soft handoff instruction.
 6. Acellular telephone that searches for a destination base station inresponse to a handoff instruction, comprising: a receiving unit operableto receive the handoff instruction from a current base station, thehandoff instruction being one of a soft handoff instruction and a hardhandoff instruction that is able to include information indicating afirst search window size; and a searching unit operable, when thereceived handoff instruction is the hard handoff instruction, to searchfor a pilot signal from a neighboring base station using a search windowthat is larger in size than at least one of the first search window sizeand a second search window size that is predetermined for the softhandoff instruction wherein the searching unit comprises: a storagesubunit that stores information indicating the second search windowsize; a judging subunit operable to judge whether the received handoffinstruction is the hard handoff instruction; and a searching subunitoperable to search for the pilot signal from the neighboring basestation, the search being performed (i) using the search window that islarger in size than at least one of the first and second search windowsizes when the judgment is in the affirmative, and (ii) using a searchwindow having the same size as one of the second search window size anda third search window size that is specified by the received handoffinstruction, when the judgment is in the negative.